Interesting article on features to look for in a good UI/screen-legible typeface. Apparently it’s pretty important that there be two-story lowercase as and gs.
I’ll leave you to read his conclusions yourselves. Though I don’t necessarily agree with all of them*, I must mention that I do agree with his recommendation of PT Sans; I’ve recently become quite enamored of this humanist sans and will probably be using it in a future incarnation of this site’s template.
*I don't think we can entirely ignore form in favor of function here, or ever. My favorites of the ones compared are Segoe UI and Ubuntu.